Memories comprise one of the largest markets for semiconductor integrated circuits. In general, a memory is a storage device that retains information or data that can be output when needed. Memory devices are often characterized under such names as high speed, high density, or non-volatile memories. A high speed memory, as its name implies, is a device having extremely fast read/write times that are useful in situations where data transfer rates are critical. A high density memory has a substantial memory size for large storage capability. The most common high density solid state memory is a dynamic random access memory (DRAM). A non-volatile memory is a memory that retains information even when power is removed and is thereby a permanent storage medium. A common non-volatile memory is FLASH memory. In general, an ideal memory has characteristics of all of the above mentioned types of memory.
FLASH memory uses charge storage in a floating gate to retain information. FLASH memories operate at relatively high voltages, running counter to the trend of reducing power supply voltages for other high density integrated circuits. Moreover, they have slow program and erase times. The ability to write or store charge in the floating gate is limited to a finite number of times that can be exceeded depending on the application. Memory failure occurs if the maximum number of writes is exceeded. FLASH memory is presently limited for high density applications because it cannot be continually scaled to smaller dimensions due to gate oxide limitations.
Another type of non-volatile memory is a magnetoresistive random access memory (MRAM). MRAM has the characteristics of an ideal memory as it is a high density memory, is scalable, requires low voltage, and has low power consumption and high speed read/write times. A magnetoresistive memory cell comprises a magnetic tunnel junction (MJT) and includes ferromagnetic layers separated by an insulating dielectric. Electrons tunnel through the dielectric, known as a tunnel barrier, from a first ferromagnetic layer to a second ferromagnetic layer. The direction of the magnetization vectors in the ferromagnetic layers determines the tunneling resistance. A zero logic state is represented when the magnetization directions are parallel which corresponds to a low tunneling resistance for the magnetic tunneling junction. Conversely, a one logic state is represented when the magnetization states are anti-parallel which corresponds to a high tunneling resistance. Typically, a magnetic vector in a first magnetic layer is fixed or pinned, while the magnetization direction of a second magnetic layer is free to switch between the same and opposite (anti-parallel) directions. The memory is non-volatile because the ferromagnetic material holds the magnetization vectors when the memory is not powered. It should be noted that the selection of the parallel state or the anti-parallel state as a logic one or zero state is arbitrary.
Typically, in MRAM and related magnetic sensor technology, the fixed layer is a pinned synthetic antiferromagnet (SAF). SAF structures are well known in literature and generally comprise two ferromagnetic layers of equal magnetic moment, separated by a spacer layer that provides antiferromagnetic coupling between them. Due to the antiferromagnetic coupling, the moments of the ferromagnetic layers point in opposite directions in the absence of an applied field. The strength of the SAF is typically expressed in terms of the saturation field Hsat, which is the field needed to force the moments of the layers parallel to each other. A unique feature of a SAF with a well-defined magnetic anisotropy is the flop behavior. As an external field increases, the moments suddenly turn perpendicular to the field when it reaches a critical value called the flop field Hflop. Practically, the moments of the ferromagnetic layers start to move in the vicinity of the flop field. Flop field is determined by the uniaxial anisotropy (Hk) of the layers and the saturation field (Hsat) of the SAF. Uniaxial anisotropy is the field needed to saturate the magnetic moment of a film along its hard axis. To increase the field where the SAF starts to move, or in other words, the moments of the layers start to move, it is a common practice in the industry to use a pinning layer. The pinning layer fixes the moment of the FM layer adjacent to it in a particular direction, which in turn sets the direction of the other layer in the SAF. Typical pinning layers used are Mn-based alloys, such as IrMn, PtMn, etc. A high temperature anneal is needed for the pinning layer to pin the FM layer adjacent to it. The pinned SAF has certain disadvantages and problems associated with it. Some of the reliability problems are associated with Mn diffusion from the pinning layer. Also, the alloy typically used as the pinning material, PtMn, is very costly and needs a very high temperature anneal for pinning, thereby increasing the thermal budget. Addition of a layer always adds complexity to the MTJ stack. A fixed SAF with no pinning layer overcomes and embodies the above mentioned problems and advantages.
The use of un-pinned SAFs as a fixed layer is disclosed in U.S. Pat. No. 5,583,725; however, the patent does not describe the specific alloys and particular material stacks that are needed for the toggle MRAM described in U.S. Pat. No. 6,545,906, that certain magnetic criteria need to be met regardless of which specific alloy is used, or that the fixed layer needs to be set in a particular direction for all the bits for proper device operation.
The role of a fixed SAF is to remain rigid while the fields are used to switch the free-layer, which can be a single free layer or a SAF (introduced by U.S. Pat. No. 6,545,906). The magnitude of the field where the free layer switches is defined as the switching field. If the fixed SAF magnetic moments rotate even slightly during switching, the switching distributions of the free layer will be broadened and memory operation will be compromised. To be magnetically rigid enough for practical use, the SAF structures should exhibit a high saturation field as well as a well-defined high flop field. A high and well-defined flop field is an important requirement that is essential regardless of which specific alloy is used, as it ensures that the magnetic moments of the fixed SAF do not rotate while the free layer is switching.
Another issue associated with a pinning-free structure is finding a way to set the direction of the fixed layer to be the same for all bits in all die on the wafer. This issue arises because, in the absence of a pinning layer, the SAF is equally likely to be in either of the two stable magnetic states with the fixed layer moment either to the right or to the left.
Accordingly, it is desirable to provide a pinning-free synthetic antiferromagnet structure incorporating all the above requirements for use in conjunction with the toggle magnetoresistive random access memory array. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.